Instruments for measuring particles in air are utilized in an increasing number of situations. Perhaps the single largest field of use is in the semiconductor industry wherein cleanliness in fabricating semiconductors is extremely important. For example, particle counters are utilized to monitor the number of particles in so called clean rooms in which semi-conductor manufacturing operations are conducted. Clean rooms are usually classified on the basis of the maximum number of particles of a given size permitted per volume of air. To meet specifications and maintain quality levels of the product being made, it is necessary to ensure that particle contamination is kept to an acceptable minimum requirement.
The basic system utilized in most airborne particle counters transmits a sample air stream through a beam of light, which results in light energy being scattered by the particles. This energy is detected and measured by suitable optics and electronics. Significant improvements have been made in the optics and electronics employed for detecting and processing the scattered light signals. Also, the advent of lasers has greatly improved the quality and intensity of the illuminating light beam.
In spite of these improvements, the accuracy of the data output from the instruments still leaves much to be desired. For example, in testing or calibrating particle counters, one technique employed is to inject a sample jet stream through the instrument that carries only particles of a known size. Output signals from such a test sample should be fairly uniform. However, it has been found that typically such results are not very uniform. Similar tests also reveal other inaccuracies, such as recirculating particles and appreciable discrepancies in absolute count correlation between different instruments. Thus, further improvements of particle counters are needed.
It is believed that one area of the instrument that has not received sufficient attention is the system for providing the sample air jet that flows into the instrument. One type of previously used nozzle is simply a tube with a circular cross section. Another type employs a tube with a circular cross section having a short tip with a generally rectangular cross section to fit within the diameter of the light beam. One nozzle of that type was made fairly precise by investment casting. In another arrangement, the end of a circular tube was pinched by a suitable tool to be less than the diameter of the light beam. A tube surrounding a nozzle for focusing or otherwise confining the jet has been used. These lack some uniformity of results. Thus, a need exists for improving the sample airflow delivery system.